1. Field of the Invention
This invention relates to an internal combustion engine piston ring having a hard film formed by ion plating on at least its sliding outer face, and a process for manufacturing the same.
2. Description of the Prior Art
In recent years, internal combustion engines for automobiles are increasingly made higher in their speed and power, resulting in an increase in thermal load around pistons. Hence, piston rings used therein are now sought to have a higher wear resistance and a higher seizure resistance.
The sliding outer faces of piston rings have been heretofore commonly treated by hard chromium plating, alloy thermal spraying or stainless steel nitriding, which have been put into use making the most of their features. Such conventional surface treatments, however, have become unsatisfactory in view of performance in some of the internal combustion engines having been made higher in speed and power.
For this reason, it is proposed in recent years to apply a PVD (physical vapor deposition) coating of a ceramic material having superior wear resistance and seizure resistance, to the sliding outer faces of piston rings, as disclosed, for example, in Japanese Laid-open Patent Applications No. 57-57868 and No. 57-65837, Japanese Laid-open Utility Model Application No. 58-35648, Japanese Utility Model Publication No. 1-22922 and Japanese Patent Publication No. 1-33658.
The ceramic material coatings or films formed by PVD, however, have had the problem that they are so hard and brittle that an attempt to make their thickness large enough to fulfill the lifetime of engines tends to cause chipping or peeling of films during their use and a deterioration of the strength of the piston rings themselves.
Moreover, as to sliding performance, although very good wear resistance and seizure resistance can be achieved in usual sliding conditions, they may wear the opposing cylinder walls when used in combination with soft cast-iron cylinders and lubricating oil is in an extremely small quantity.